(a) Field of the Invention
The present invention relates to a super absorbent polymer and a preparation method thereof, and more particularly, to a super absorbent polymer having excellent liquid permeability without a reduction in other physical properties, and a preparation method thereof.
(b) Description of the Related Art
A super absorbent polymer (SAP) is a type of synthetic polymeric materials capable of absorbing moisture from 500 to 1000 times its own weight. Various manufacturers have denominated it as different names, such as SAM (Super Absorbency Material), AGM (Absorbent Gel Material), etc. Since such super absorbent polymers started to be practically applied in sanitary products, now they have been widely used not only for hygiene products such as disposable diapers for children, etc., but also for water retaining soil products for gardening, water stop materials for the civil engineering and construction, sheets for raising seedling, fresh-keeping agents for food distribution fields, materials for poultice or the like.
As a preparation process for such super absorbent polymers, a process by a reverse phase suspension polymerization and a process by a solution polymerization have been known. For example, Japanese Patent Laid-open Publication Nos. S56-161408, S57-158209, and S57-198714 disclose the reverse phase suspension polymerization. The process by the solution polymerization further includes a thermal polymerization method in which a polymerization gel is polymerized while being broken and cooled in a kneader equipped with a plurality of shafts, and a photo-polymerization method in which an aqueous solution with a high concentration is irradiated with UV rays onto a belt to be polymerized and dried at the same time. The hydrogel polymers thus obtained through the polymerization reaction are generally marketed in a powdery form after drying and pulverization processes.
Meanwhile, in the preparation process of the super absorbent polymer, a water-soluble component which is an uncrosslinked polymer is produced. A high content of the water-soluble component increases a liquid absorption property of the super absorbent polymer. However, when the super absorbent polymer is in contact with a liquid, it is easily eluted to make the surface sticky or to cause an unfavorable effect when being in contact with the skin. Further, if the content of the water-soluble component is high, the eluted water-soluble component generally exists on the surface of the super absorbent polymer and it makes the super absorbent polymer sticky, leading to a reduction in liquid permeability which is an ability to rapidly transfer a liquid to other super absorbent polymer.
Accordingly, there is a demand to develop a super absorbent polymer having excellent liquid permeability while maintaining high water absorption property.